The Institute of Electrical and Electronics Engineers (IEEE) has adopted a set of standards for wireless local area networks (LANs), known as 802.11. Wireless products satisfying 802.11a, 802.11b, 802.11g, and 802.11n are currently on the market.
A new generation of mobile and handheld devices has emerged, supporting multiple wireless interfaces or radios. A single mobile device, for example, may have three or four radios, each one supporting a different wireless network, such as wireless wide-area network, or WWAN (cellular), wireless local-area network, or WLAN (802.11a/b/g/n), wireless personal-area network, or WPAN (Bluetooth, UWB, Zigbee), and wireless metropolitan-area network, or WIMAX (802.16). The flexibility built into such devices is intended to maximize wireless connectivity and user experiences.
Wireless devices under the 802.11n standard are expected to support legacy standards, such as 802.11a, 802.11b, and 802.11 g. TGn is working to support data rates of up to 300 Megabits per second (Mbps) for two spatial streams and 450 Mbps for three spatial streams.
Supporting these higher data rates is difficult to achieve. For one thing, with wireless transmission, acknowledgement (ACK) packets must be sent between devices. This is because, in contrast to wired transmissions, loss rates may be quite high, 20%, 30%, even 50%, for some transmissions. Thus, device A transmits a packet to device B. The ACK to device A, in essence, informs device A that device B received the original packet.
The IEEE 802.11n specification establishes very tight timing between the time the device B receives the packet and subsequently transmits the ACK packet. This time interval is known as a short inter-frame spacing (SIFS) interval. For 802.11n, as the data rates increase, the packets are transmitted at higher rates, the packet takes more time to decode, but the SIFS interval does not change. There are two reasons for this. The training field of high-throughput frames that helps the DSP estimate the channel is positioned immediately before the payload, delaying the start of decode, relative to legacy frames. Further, the Viterbi decoder inherently works at one bit or two bits per clock. Secondly, making the Viterbi decoder faster requires either a faster clock or more hardware. For TGn rates, the amount of bits per microsecond is higher (˜300 bits) compared to legacy rates (54 bits), thus considerable delay is added. Whether the frame is TGn or legacy, the receiving device, device B, should not send an ACK to device A until the received packet has been deemed correct, this may be done only after the last byte of the received frame has been decoded.